cứ

The adverb cứ is highly frequent in everyday Vietnamese and has several distinct but related uses:

1. Permission and Encouragement (Go ahead)
When you want to tell someone to "feel free to" or "go right ahead" without hesitation, place cứ before the verb (and directly after the subject, if one is present). To make the invitation sound warm and polite, it is almost always paired with a final particle like đi, nhé, or nha at the end of the sentence. You will also frequently hear the compound cứ việc, which explicitly means "feel free to" or "just go ahead and do it". Another common phrase is cứ để, meaning "just let" (e.g., just let me do it), as well as the highly common set phrase cứ tự nhiên (make yourself at home / act naturally).

2. Persistence and Continuation (Keep on)
When describing an ongoing action, cứ indicates that the action continues persistently, habitually, or despite obstacles and contrary advice. In this context, it often carries a nuance of stubbornness or defiance. You will frequently hear the phrase cứ thế (just like that / continuing in that manner) or vẫn cứ (still keep on), which emphasizes that an action continues despite contrary conditions.

3. Reliable Habits (Whenever / Every time)
The structure cứ ... là ... is used to express that whenever one thing happens, another thing inevitably follows. This is often paired with hễ to form hễ cứ ... là ... for extra emphasis.

4. Frequency and Intervals (Every / Per)
When placed before a time interval or quantity, cứ means "every" or "per". For example, you can say cứ 5 phút (every 5 minutes) or cứ mỗi ngày (every day) to describe how often something occurs. It is also frequently paired with reduplicated time nouns for habitual emphasis, such as cứ chiều chiều (every afternoon) or cứ đêm đêm (every night).

5. Prepositional Use (According to / Based on)
Cứ can also function as a preposition meaning "according to" or "based on" (e.g., cứ theo kế hoạch - according to the plan). A related structure is cứ + [Noun] + mà + [Verb], which means using the noun as the basis or direction for the verb (e.g., cứ đường này mà đi - just follow this road and go). You will also see the formal compound căn cứ (vào), meaning "based on" or "pursuant to".

6. Common Compounds
As you become comfortable with the language, you will also frequently encounter cứ in essential compounds such as bất cứ (any / whatever), cứ như là (as if), and cứ cho là (let's assume that / even if).

Important Note on Politeness:
Because beginners often learn cứ in the context of warm invitations, there is a myth that the word itself is inherently polite. In reality, the politeness comes entirely from your tone of voice and the final particles you use. When describing third parties, cứ frequently carries an exasperated or negative nuance (e.g., "He just keeps talking!").